1. Field of the Invention
The invention relates to a method for connecting objects by means of at least one plastically deformable connecting body, in which the connecting body is first of all heated and is subsequently permanently deformed by means of a shaping punch. The invention also relates to an apparatus for carrying out the method.
2. Description of the Related Art
In a known method according to EP 0 965 400 A2, the connecting body, which constitutes a projection of one of the two parts which are to be connected, is first of all heated by means of a hot-air nozzle, after which a cold punch is then used to deform the connecting body which reaches through a second part which is to be riveted on and connects the latter to the first part by means of the integrally formed rivet head.
Riveting methods of this type require a high use of energy, since the hot air flowing out of the nozzle not only heats up the connecting body, i.e. the rivet, but also the surrounding material, which may sometimes lead to visual impairments or even to damage to the material. In addition, the connection is frequently not characterized by a particularly good press fit, which is generally attributed to the non-uniform heating up of the projection, connecting body or rivet.
The object of the invention is to provide a method and an apparatus for connecting objects by means of plastically deformable connecting bodies with the aid of which high-strength, thermal rivet connections can be obtained in an energy-saving manner without impairing the material surrounding the connecting body.
According to the invention, a heated gas flow which flows closely around the connecting body is produced in order to uniformly heat the connecting body prior to deformation. The heating action is ended at the latest directly after the deformation, depending on the heat contents of the system comprising the connecting body and shaping punch, and a cool gas flow is maintained until the deformed connecting body and the shaping punch are cooled.
By means of the hot gas flow which flows closely around the connecting body, not only is completely uniform heating of the connecting body, and therefore a particularly good prerequisite for the subsequent deforming and riveting process, obtained, but also the heating up of the material surrounding the connecting body is avoided, which is of particularly significant importance if the materials in this case are temperature-sensitive materials. The uniform heating through of the connecting body is an essential prerequisite for a homogenous material structure after the deformation, i.e. after the riveting, so that the rivet connection obtained by this means is distinguished by high strength and withstands high loading forces. The time for ending the heating action depends on the strength of the heat contents of the entire system formed from the connecting body and the shaping punch. If, for example, the operation is carried out with a cold shaping punch and as a result there is the risk of the connecting body cooling too rapidly, depending on its mass, if the heating action is shut off relatively early, then the heating action inevitably has to be maintained for somewhat longer than if the heat conditions are more favourable, thus enabling the heating action to be switched off somewhat earlier so as to shorten the cycle times.
The expression xe2x80x9cconnecting bodyxe2x80x9d is to be understood as either a loose rivet which can be introduced into a hole of the two parts to be connected or as a projection on one part which can be passed through a hole in the part to be fastened on.
The gas flow necessary for uniform heating of the connecting body is heated by a heating element provided in the region of the connecting body.
In order to cool both the shaping punch and the rivet head which is formed, in a further development of the invention the gas flow is increased in terms of quantity after the end of the heating action. By this means, the cooling action is accelerated.
In a first variant of the method, the shaping punch is heated for the deformation and the gas flow is heated by the shaping punch.
In order for the shaping punch to be able to heat the gas flow as intensively as possible, in a further development of the invention provision is made for the gas flow at the upper end of the connecting body to be directed downwards and to be restricted outwards by a wall. It is thereby possible to guide the gas flow very closely and at high speed around the shaping punch and as a result to obtain a good heat transfer between the shaping punch and gas flow. In addition, the restriction by means of a wall has the advantage that a small, heated quantity of gas which is guided very closely around the deformable connecting body suffices, as a result of which little energy is required for the uniform heating up and plasticization. In addition, outer regions of the parts to be connected are not affected detrimentally.
In order to obtain these advantages in a particularly impressive form, it is provided, in an expedient refinement, that the wall surrounds the shaping punch, at least in the region of its heating, with little clearance and the gas flow is introduced into the gap between the shaping punch and wall from above in the direction of the free end of the shaping punch.
The gas flow is preferably accelerated in the direction of the free end of the shaping punch.
In a second variant of the method, provision is made for the wall to be designed as a radiating element and to heat both the deformable connecting body and the gas flow. In this type of method, it is also essential that the gas flow contributes to the uniform heating up of the connecting body, since in most cases the radiating heating element cannot be designed so uniformly with respect to its radiated heat that a completely uniform heating of the connecting body would be ensured without a gas flow.
In this second method variant, the gas flow is guided through a channel within the shaping punch and, emerging at the free end thereof, is directed onto the deformable connecting body. By this means, uniform flow around the connecting body, and therefore a uniform distribution of temperature within this connecting body, is ensured.
The apparatus, in particular for carrying out the method, having a shaping punch for deforming a deformable connecting body, a heating means for heating the connecting body and a device for supplying a gaseous medium to the connecting body, is characterized, in a first variant, in that the shaping punch is held, and guided in a vertically displaceable manner, at least with its heatable, lower, free end in a flow-restricting means for the gaseous medium, in which case the medium can be introduced into the gap between the shaping punch and flow-restricting means.
In this design variant, the gaseous medium is heated by the shaping punch, which contributes to the evening out of the temperature in the deformable connecting body, after which this connecting body is deformed by means of the heated shaping punch.
The reference to the fact that the apparatus is provided in particular for carrying out the method means that this apparatus can be used without any structural change not only for deforming connecting bodies, but also for deforming objects which are not used for connecting parts, but rather only have to be deformed in their design, for example at the end of a projection.
In a preferred refinement of the invention, the shaping punch is guided together with the flow-restricting means in a vertically displaceable manner.
In order to obtain an effective use of energy for heating the shaping punch and the gaseous medium, in a development of the invention the shaping punch is equipped in the interior with an electric heating means and is surrounded, with little clearance, by a flow-restricting means which has a gas supply line and conducts the gas flow to the deformable connecting body. The electrically heated shaping punch heats the gas medium which is introduced into the gap between the shaping punch and flow-restricting means and is heated rapidly and with little expenditure of energy because of the relatively narrow gap.
A preferred refinement of this apparatus is characterized in that the electric heating means is arranged, as a heating cartridge, in the direct vicinity of the front end of the thin-walled, low-mass shaping punch. The formation of the shaping punch with a thin wall and therefore with a small mass has the advantage that the heating of the shaping punch can take place with little energy and that a rapid temperature change between heating and cooling is possible. Since the heating energy is deployed in the direct vicinity of the end used for deforming the connecting body, energy is thereby saved and the heating and cooling processes take place more rapidly.
It may be advantageous, particularly when relatively large amounts of gas are used, if, in a development of the invention, the shaping punch is provided on its outside in the heating region with heat-transfer ribs, since by this means, in particular in conjunction with a narrow clearance for the flow-restricting means, a very good heat transfer between the shaping punch and the gas which is to be heated up is obtained.
If the flow-restricting means is tapered in a nozzle-like manner at its lower, open end, then the gaseous medium is accelerated in the direction of the connecting body to be heated, which increases the heat transfer. This refinement is particularly advantageous if the heating punch is provided with heat-transfer ribs. The tapering then brings about a relatively small outlet cross section, with the result that the flow-restricting means is at a very small distance from the connecting body at the outlet end. By this means, not only is particularly good heating of the connecting body obtained, but also the material surrounding the connecting body is shielded against the effect of heat.
According to a second design variant, the flow-restricting means is designed as a radiating heating means surrounding the shaping punch and the gas supply is formed by a channel which is provided within the punch and opens out at the free end of the punch. By this means, the gas flow is directed from above onto the connecting body, flows around the latter and is heated by the radiating heating means, with the result that the gas flow contributes to evening out the heating of the connecting body. In addition, the gas supply within the shaping punch is used for cooling the latter, in order to reduce the cycle times, since the said shaping punch is heated up on the heated connecting body in the deforming process.
The shaping punch can be displaced relative to the radiating heating means, so that it only passes into a heated region during the deforming process.